Simulator sickness

Simulator sickness (SSS for simulator sickness syndrome) is a problem that may occur in all virtual environments, both in simulators, driving games and virtual reality applications, although in the latter case it is often referred to as cybersickness. The most common symptoms are general discomfort, headache, stomach awareness, nausea, vomiting, pallor, sweating, fatigue, drowsiness, disorientation, and apathy, see the Wikipedia entry. The most important factor that induces simulator sickness is a mismatch between visual cues of acceleration and vestibular cues of acceleration. During driving in a regular car, the driver senses accelerations and deceleration via the sensors in the inner ear, the vestibular system. In a static simulator these are absent, while the visual cues are perceived, and this mismatch is believed to be the reason why people get these symptoms.

Most simulators are static simulators, without a motion platform or moving base. The reason is that motion platforms are very expensive. Sometimes it is concluded from the above reasoning, that static simulators are worse than simulators with a motion platform, because in a moving base simulator the drivers would experience motion cues from their vestibular system, and as a result, the mismatch would be smaller. Yet, its not that simple. Also very expensive simulators with motion platforms sometimes have problems with SSS. It depends on a lot of factors. If you let experienced police officers or ambulance personnel, who are all very familiar with strong motion cues in real driving, drive in an expensive moving base simulator with complex scenarios, you can be sure that simulator sickness symptoms will occur nonetheless. The motion cues are very difficult to replicate in a motion platform and also imperfect motion cues result in a mismatch.

Individual differences

Some people are more susceptible to simulator sickness than others. Young and inexperienced drivers suffer the least. This makes a driving simulator a good instrument for driver training, because the students are typically young and inexperienced. A possible reason for this is that these people have not yet learned the associations between what they see and the motions they feel, so the mismatch is a lot smaller. Also, age may play a role. Older people are much more sensitive to simulator sickness, especially if they have more driving experience.

Drivers with a lot of driving experience have the highest risk of experiencing SSS symptoms. This is why driving instructors often complain about driving simulators and, because of the symptoms, have difficulties with driving in a simulator.They often feel embarrassed because of that and as a result may question the validity of driving simulators. It is important to note, however, that driving is mainly a visual task, and all driving skills can be learned perfectly without vestibular motion cues.

In tests of driver fitness, older people or people with neurological pathology are typically included, and this is a group that is much more likely to experience problems with simulator sickness. That’s why it is very important that measures are being taken to reduce the likelyhood of SSS in tests of fitness to drive for these groups.

Accelerations

When driving at a constant speed, SSS symptoms will not occur. The reason is ofcourse that there are no motion cues coming from the vestibular system. There are only visual cues that indicate the driver is proceeding at a certain speed. Because no vestibular cues are expected, there’s no mismatch and as a result no symptoms of SSS.

However, although in the case of a constant speed there are no longitudinal accelerations or decelerations, there may well be lateral accelerations, when the road is curved. These lateral acceleration are experienced as a certain g-force (a gravity like force) that are experienced sideways: in the real world the driver would feel a pull to left or right, instead of a pull forward or backward, in the case of longitudinal accelerations or decelerations. If you are driving on a mildly winding road, with a constant speed, the lateral accelerations are only mild, so the expected motion cues are also mild, and the mismatch will be rather low. However, when the driver needs to make 90 degree turns, as when turning left or right at an intersection, expected lateral acceleration motion cues are a lot stronger, and the experienced mismatch in the simulator is stronger too. So, for vulnerable groups, these kinds of maneuvers are not recommended. For older and/or more experienced drivers it is recommended to make scenarios that consist on driving on straight or mildly curving roads and avoid making sharp curves.

Visual motion cues

The more immersive the experience, the higher the likelyhood of simulator sickness. So, a better and more realistic simulation with surround views on large screens gives a higher risk of SSS symptoms compared to a simple simulation on a single monitor. The probable reason is that in the former case the visual motion cues of accelerations are much stronger, resulting in a larger mismatch, and as a result a higher risk of symptoms.

The trick is to find a balance between a realistic impression of driving, which includes sufficient visual cues of speed and acceleration/deceleration, while limiting these visual cues as much as possible. So, there have to be visual acceleration cues but they must not be larger than needed. If the cues are too strong, SSS symptoms will increase because of a larger mismatch, but if they are too weak, the experience of driving may suffer.

There are many situations where 180 degrees of vision (with 3 displays for rendering) is not really required. In tests of fitness to drive with older subjects, it may be considered to only use a single monitor. If you don’t need to look into side streets for traffic, that would be an acceptable solution, although speed perception benefits strongly from a wider field of view. In a single-monitor solution with a horizontal field of view of, say, 70 degrees, peripheral vision is lacking, and peripheral vision contains both strong visual cues for speed and for accelerations. So this will reduce the risk of SSS symptoms. A similar way to reduce the strength of visual acceleration cues is by lowering the brightness of the left and the right monitors, because this will reduce the strength of visual peripheral cues.

Display size probably affects the strength of visual acceleration cues as well, so from that perspective, larger displays also result in a higher risk of SSS symptoms. Visual complexity and detail has similar effects: don’t make the scenes too complex because that increases the strength of visual cues.

With the graphics boards and computers becoming better and cheaper, people expect high quality graphics with surround views of over 180 degrees horizontal field of view (FOV), preferably on large displays or projections. They argue that a racing game also has high quality graphics so they expect the same of driving simulators. But a racing game normally uses a limited field of view on one display, so simulator sickness is much less of an issue in games. But even so, with graphics becoming more realistic and the images more detailed, even in one-display games, the visual motion and acceleration cues become strong enough to induce SSS symptoms in those games.

With driving simulators with over 180 degrees horizontal FOV, the risk of symptoms is much higher, and explicit measures must be taken to reduce the risk of simulator sickness.